1. Field of the Invention
The present invention relates to image processing circuits and image processing methods for image reduction of black-and-white images in such as facsimile apparatuses, digital copying machines, image scanners, and printers that can prevent loss of thin lines and generation of solid black images and also prevent image quality degradation in inverted (rotated 180°) T-shaped images and line thickening in laid (rotated 90°) T-shaped images, which cannot be solved by conventional operations.
2. Description of the Related Art
As for image processing techniques for reducing black-and-white images, generally, the following processes are carried out as the simplest processes: a simple thinning process of selecting one arbitrary line from among focused N lines and rendering the selected line as an output image, and an OR process of rendering a logical sum (OR) of all the N lines as an output image.
In the simple thinning process, however, when a line to be thinned out includes a black pixel and a line including only white pixels is left, for example, a thin line in the main scanning direction is missed. On the other hand, in the OR process, it is possible to prevent a thin line in the main scanning direction from being missed. However, at the same time, if there is one black pixel in N lines, it is determined that the output image at the column position is formed by black pixels. As a result, there is a disadvantage in that the output image is more likely to be a solid black image.
Therefore, as described in Japanese Laid-Open Patent Application No. 8-289137, an image reducing method of using the simple thinning process and the OR process together and selectively carrying out one of the processes in accordance with the quality of an image is known. In addition, a reducing method of selectively carrying out a thinning process according to the number of black pixels is also known.
In a pixel density conversion apparatus using the above-described methods, for example, the OR process is performed on image data of character documents so as to prevent thin lines from being missed, the simple thinning process is performed on image data of photographic documents so as to prevent degradation in reproducibility of halftones, and, in the thinning process according to the number of black pixels, the number of black pixels of each line is counted and lines other than the line having the largest number of black pixels are thinned out so as to prevent thin lines from being missed.
However, such processes do not solve the problem that an output image becomes a solid black image when the OR process is selected. Thus, fundamental solutions for the above-described problems are not provided.
Additionally, the thinning according to the number of black pixels provides an output image of the middle-level between the simple thinning process and the OR process. Thus, the thinning according to the number of black pixels only slightly reduces loss of thin lines and production of solid black images, and does not provide the ultimate solutions.
By the way, methods for compensating for the shortcomings of the OR process include methods of referring to data of a peripheral pixel that is adjacent in the sub-scanning direction to a focused pixel of a reduced image (image after reduction), when performing a reducing process in the sub-scanning direction, as shown in the following Equations 1 and 2.o(x, y)=inv[o(x, y−1)]·i(x, 2y−1)+i(x, 2y)  Equation 1oro(x, y)=inv[o(x, y−1)]·i(x, 2y)+i(x, 2y+1)  Equation 2                i: pixel data of original image                    (black=1, white=0)                        o: pixel data of reduced image                    (black=1, white=0)                        x: pixel value in main scanning direction        y: pixel value in sub-scanning direction        
Hereinafter, the reducing process of Equation 1 is referred to as a “process using conventional technique A”, and the reducing process of Equation 2 is referred to as a “process using conventional technique B”.
FIGS. 1B, 1C, and 1D show the results of performing, on a horizontal-striped input image shown in FIG. 1A, a 50% reducing process by an OR operation, the process using conventional technique A (50% reducing process), and the process using conventional technique B (50% reducing process), respectively. As can be seen from FIG. 1B, a solid black image is produced in the reduction by the OR operation. In addition, as shown in FIGS. 1C and 1D, in the processes using conventional techniques A and B, a solid black image is partially produced.
In addition, FIGS. 2B, 2C, and 2D show the results of performing, on an inverted T-shaped input image shown in FIG. 2A, the 50% reducing process by an OR operation, the process using conventional technique A (50% reducing process), and the process using conventional technique B (50% reducing process), respectively. As can be seen from FIGS. 2B and 2C, there is no defect (good results are obtained) in the reduction by the OR operation and the process using conventional technique A. On the other hand, as shown in FIG. 2D, in the process using conventional technique B, a blank part (missing part)(denoted by the reference numeral 100 in FIG. 2D) is partially produced.
Further, FIGS. 3B, 3C, and 3D show the results of performing, on a laid T-shaped input image shown in FIG. 3A, the 50% reducing process by an OR operation, the process using conventional technique A (50% reducing process), and the process using conventional technique B (50% reducing process), respectively. As can be seen from FIGS. 3B and 3C, in the reduction by the OR operation (FIG. 3B) and the process using conventional technique A (FIG. 3C), line thickening occurs. As shown in FIG. 3E, in the process using conventional technique B, there is no defect (a good result is obtained).